Remove deprecated neon animation, use CSS keyframe animations

[valdrinkoshi]

Remove neon-animation dependency in favor of CSS keyframe animations, taking a similar approach as in https://github.com/PolymerElements/iron-dropdown/pull/154 and PolymerElements/paper-dialog#163.

The expand/collapse animation is done in a performant way using transform, which doesn't cause layout thrashing - more details here https://medium.com/@valdrinkoshi/performant-expand-collapse-animations-93d99e80f7f

The properties openAnimationConfig and closeAnimationConfig are now deprecated, as paper-menu-button cannot really allow users to define animations for its internal iron-dropdown, as there is no way to pass @keyframes into its shadowRoot. Because of this, paper-menu-button doesn't offer a way to customize open/close animations :[

Nevertheless, after testing this on internal projects it doesn't break them - as most of them rely on the default animations setup by paper-menu-button.

This PR depends on https://github.com/PolymerElements/iron-dropdown/pull/154.